The present invention relates to a technique for spreading a signal spectrum.
Techniques of varying the frequency of a signal to suppress electromagnetic interference (EMI) noise included in a signal are known. JP-B-7-85524 discloses a technique of producing an output signal by generating a triangular wave which is frequency-modulated according to a random signal and PWM-amplifying an input signal according to the generated triangular wave.
JP-A-2004-282714 discloses a technique of producing an output signal by generating, with a PLL circuit, a triangular wave which is synchronized with a frequency-modulated reference signal and PWM-amplifies an input signal according to the generated triangular wave.
Incidentally, where frequencies are selected according to a random signal, every frequency is designated as the next frequency at the same probability and some deviation may occur in designated frequencies in the short term. For example, where one of five frequencies (first to fifth frequencies) is selected randomly each time, the probability that the first frequency will be selected next is 20% even if the current frequency is also the first frequency. That is, although all frequencies occur at the same probability in the long term, a frequency deviation may occur in the short term. That is, the related techniques have a problem that EMI noise in an output signal may increase due to frequency deviation.
Furthermore, to spread the spectrum of an output signal, the related techniques require the frequency modulation circuit or the PLL circuit, which raises another problem that the circuit configurations are complex.